1. Field of the Invention
The present invention relates to an image pickup apparatus and, more particularly, to a color image pickup apparatus having signal compensation means for compensating an image signal in correspondence to color temperature changes of an image illumination source.
2. Description of the Related Art
Generally, in color television (TV) cameras, there are provided white balance control means and compensation means for compensating an image signal in correspondence to color temperature changes of an illumination source so that the signal becomes equivalent to one which is obtained under the illumination of a standard light source.
Moreover, though the exposure time of an image pickup tube or element for one TV field is fixed at, for example, 1/60 second under the NTSC system (1/50 second under the PAL system) in the conventional TV cameras, it has been proposed to equip the camera with means for setting the exposure time at a time period shorter than one field period (1/60 sec.). This technique enables a VTR (Video Tape Recorder) to reproduce a clear still picture without shaking of the images from scenes of rapidly moving objects recorded on the tape.
In FIG. 1, there is shown a circuit system for a TV camera equipped with such a means.
In the Figure, an image forming lens 1 is provided for forming an image of an object. A color image pickup element 3, e.g. a CCD (Charge Coupled Device), is provided with a color filter and is arranged to receive the object image formed by the lens 1. A shutter 2 is disposed between the lens 1 and the pickup element 3 to control the exposure of the pickup element 3 to the image light.
An amplifier circuit 4 is connected to receive the output of the pickup element 3. A color demodulation circuit 5 is connected to demodulate three primary color component signals R (Red), G (Green) and B (Blue) from the output of the amplifier circuit 4 in a known manner in response to a clock signal CK supplied from a control circuit 20. A luminous signal forming circuit 6 is connected to form a luminous signal Y based on the color component signals R, G and B output from the color demodulation circuit 5.
Gain controllable amplifier circuits 7 and 8 are connected to receive the color component signals R and B output from the color demodulation circuit 5, respectively. A subtraction circuit 9 is connected to subtract the output Y of the luminous signal forming circuit 6 from the output R of the amplifier circuit 7. Another subtraction circuit 10 is connected to subtract the output Y of the luminous signal forming circuit 6 from the output B of the amplifier circuit 8. A white balance control circuit 11 is connected to control the gain of the amplifier circuit 7 based on the output of the subtraction circuit 9 at the time of the white balance setting. Another white balance control circuit 12 is connected to control the gain of the amplifier circuit 8 based on the output of the subtraction circuit 10 at the time of the white balance setting. Signal level adjusting circuits 14 and 15 are connected to adjust the signal levels of the outputs of the subtraction circuits 9 and 10 based on level adjusting signals supplied from outputs C.sub.1 and C.sub.2 of a color temperature setting circuit 16, respectively. The color temperature setting circuit 16 is arranged to produce the level adjusting signals in accordance with a set color temperature. A modulation circuit 17 is connected to modulate a color subcarrier (fsc) with the outputs of the level adjusting circuits 14 and 15 according to a predetermined standard.
A luminous signal processing circuit 13 is connected to process and add synchronization signals SYNC. to the output of the luminous signal forming circuit 6 in a known manner. An addition circuit 18 is connected to add the outputs of the processing circuit 13 and the modulation circuit 17 to form a composite color TV signal.
A shutter drive circuit 19 is connected to drive the shutter 2. The control circuit 20 is connected to drive and control the image pickup element 3 and the drive circuit 19. The control circuit 20 may comprise a micro-computer and an oscillator. An exposure time setting circuit 21 is connected to set an exposure time for the shutter 2, to the control circuit 20.
In operation, the image pickup element 3 produces a color image signal when exposed to image light coming through the lens 1, and the open shutter 2. The image signal is repeatedly read out from the pickup element 3 by the control circuit 20 at a predetermined cycle (60 cps for the NTSC system). The output of the pickup element 3 is amplified in the amplifier circuit 4 and is supplied to the demodulation circuit 5 which demodulates the color component signals R, G and B from the output of the amplifier circuit 4 based on the clock signal CK supplied from the control circuit 20. These color component signals R, G and B are supplied to the luminous signal forming circuit 6 which forms the luminous signal Y based on the received color component signals R, G and B and according to the following equation: EQU Y=0.30R+0.59G+0.11B
The luminous signal Y formed by the forming circuit 6 is supplied to the subtraction circuits 9 and 10 and to the processing circuit 13. The processing circuit 13 adds the synchronization signals SYNC from the control circuit 20 to the luminous signal Y and supplies the resulting signal to the addition circuit 18.
On the other hand, the color component signals R and B are respectively supplied through the amplifier circuits 7 and 8 to the subtraction circuits 9 and 10. These latter circuits subtract the luminous signal Y from the color component signals R and B to produce color difference signals R-Y and B-Y, respectively. Here, as is well known in the art, the white balance control circuits 11 and 12 respectively control the gains of the amplifier circuits 7 and 8 so that the levels of the color difference signals R-Y and B-Y produced by the subtraction circuits 9 and 10 become zero with respect to a white picture.
The color difference signals R-Y and B-Y are adjusted by the signal level adjusting circuits 14 and 15 based on level adjusting signals supplied in correspondence to the color temperature from the outputs C.sub.1 and C.sub.2, respectively, of the color temperature setting circuit 16. The color temperature and accordingly, the level adjusting signals are set in the setting circuit 16 in accordance with the object illumination light source. Thus, it becomes possible to correct the poorness in the reproducibility of a color vector due to the difference in the color temperatures of the light sources such as a fluorescent lamp, an incandescent lamp and a sun light.
After adjustment by the signal level adjusting circuits 14 and 15, the color difference signals R-Y and B-Y are supplied to the modulation circuit 17. The modulation output of the modulation circuit 17 is supplied to the addition circuit 18 which in turn produces the composite color TV signal based on the outputs of the processing circuit 13 and of the modulation circuit 17. The thus produced composite color TV signal can be recorded by the VTR or monitored by a monitor device.
In a normal usage, the shutter 2 is kept opened under the control of the control circuit 20 by the setting through the exposure time setting circuit 21 so that the exposure time of the pickup element 3 for one field becomes equal to one field period.
On the other hand, for special purposes, the exposure time can be set at a time period shorter than one field period through the setting circuit 21.
FIGS. 2A to 2C show a shutter timing arrangement where the exposure time for one field is set at a time period of T.sub.2 which is shorter than the field period of T.sub.0.
In this case, the control circuit 20 causes the pickup element 3 to output the image signal in synchronism with a vertical synchronization signal fv (FIG. 2A), but delays the exposure timing by a time period T.sub.1 (FIG. 2B) so that the exposure period T.sub.2, (which begins at the end of the delay period T.sub.1), ends immediately before the commencement of the next field (FIG. 2C). Thus, the shutter drive circuit 19, under the control of the control circuit 20, drives the shutter 2 to expose the pickup element 3 at the time, and for the period, of T.sub.2 shown in FIG. 2C.
The technique of controlling the exposure time of the image pickup element for a time period shorter than the field period is also important in still video cameras in which a video signal of one field or one frame is recorded on a magnetic video floppy disc during one rotation thereof.
In FIG. 3, there is shown a circuit system of a still video camera equipped with a shutter control means which controls the exposure time as described above.
In FIG. 3, the elements shown by the numerals used in FIG. 1 are the same in their structure and function as those shown in FIG. 1; and elements shown by the numerals used in FIG. 1, but with primes, correspond to those shown in FIG. 1.
As shown in FIG. 3, a magnetic video floppy disc 27 is mounted on an output spindle 23a of a disc rotating motor 23. A motor drive circuit 24 is connected to drive the motor 23 under the control of a control circuit 28.
A recording circuit 25 is connected to process the output of the addition circuit 18 into a recording signal. A magnetic head 26 is connected to record the output of the recording circuit 25 onto the disc 27.
A pulse generator 22 is arranged to generate a pulse signal PG at a predetermined rotation phase within each rotation of the disc 27. The pulse generator 22 is connected to supply the pulse signal PG to the motor drive circuit 24 and to the control circuit 28.
A trigger circuit 29 is connected to trigger the control circuit 28 to initiate a picture taking and recording operation.
Other than the above, the structure of the system is the same as that of the system shown in FIG. 1.
The operation of the system will now be explained with reference to FIGS. 4A to 4I.
When a power source for the system is turned on, the control circuit 28 begins to produce the synchronization signals SYNC including the vertical synchronization signal fv with a field period of T.sub.0 (FIG. 4A). Then, when triggered through the trigger circuit 29 at a timing t.sub.1 as shown in FIG. 4B, the control circuit 28 causes the motor drive circuit 24 to drive the motor 23 as shown in FIG. 4C. The motor drive circuit 24 drives the motor 23 based on the reference timing signals supplied by the control circuit 28, the pulse signal PG supplied by the pulse generator 22 and a frequency signal supplied by and indicative of the rotation speed of the motor 23 so that the motor rotates the disc 27 at a predetermined speed of 3,600 rpm and at a predetermined phase relationship relative to the vertical synchronization signal fv. The motor drive circuit 24 produces a servo-lock signal at a timing t.sub.2 as shown in FIG. 4D when the rotation speed of the motor 23 has reached the predetermined speed of 3,600 rpm and the pulse signal PG has become synchronized with the vertical synchronization signal fv. In response thereto, the control circuit 28 internally produces a window pulse having a duration slightly longer than the field period of T.sub.0, as is shown in FIG. 4E. Then, the control circuit 28 internally produces a delay pulse at a timing t.sub.3 as shown in FIG. 4F in response to the first vertical synchronization signal fv falling in the duration of the window pulse of FIG. 4E. The delay pulse extends to a time t.sub.4 as is shown in FIG. 4F. The control circuit 28 at the time t.sub.4 causes the shutter drive circuit 19' to open the shutter 2' for an exposure period from the time t.sub.4 to a time t.sub.5 as shown in FIG. 4G. This exposure period is predeterminately set through the exposure time setting circuit 21' and corresponds to the remaining portion of the field period T.sub.0, taking therefrom the delay period from the time t.sub.3 to the time t.sub.4. At the time t.sub.5, the control circuit 28 triggers the recording circuit 25 which in turn records through the head 26 onto the disc 27 while forming a circular track thereon. This recording of the signal supplied from the addition circuit 18 is carried out for the field period from the time t.sub.5 to a time t.sub.6 as shown in FIG. 4H. At the time t.sub.6, the control circuit 28 produces a termination pulse as shown in FIG. 4I and deactivates the motor drive circuit 24 as shown in FIG. 4C.
Other than the above, the operation of the system of FIG. 3 is the same as that of the system shown in FIG. 1.
The above described cameras of FIG. 1 and FIG. 3 can operate satisfactorily when the object whose picture is being taken is illuminated from a light source whose color temperature does not change in time, e.g., sun light, an incandescent lamp, or the like.
However, in the case of an illuminating light source whose color temperature changes in time over a certain period, e.g., a fluorescent lamp, a problem may arise if the exposure time is short. That is, in the case where the exposure time is no shorter than the field period (1/60 sec. in NTSC system), e.g., 1/30 sec., 1/15 sec. or longer, the system works well with a color temperature fixedly set in the color temperature setting circuit 16. This is because the periodic change in the color temperature of the light source is averaged for the exposure time. However, in the case where the exposure time is shorter than the field period, e.g., 1/250 sec., 1/500 sec. or shorter, the color temperature for the exposure period changes depending upon the phase of the exposure timing relative to the cycle of the light source color temperature variations. In this case, the color temperature may vary in the order of a few hundred degrees in actual measurement values. Under this condition, if the color temperature is fixedly set in the color temperature setting circuit 16, the hue becomes different in each field or picture frame and the picture images become unsightly.